In producing articles of steel or other metal by this known method, a heat-destructible insert or core having the shape of the desired casting and termed a pattern is imbedded in a mass of free-flowing refractory particulate matter such as sand, the pattern consisting of a preferably thermoplastic low-melting material such as foam polystyrene which burns or vaporizes under the heat of the molten metal introduced into its site. As the vapors diffuse through the surrounding sand, the metal completely fills the void left in the mass and, upon hardening of the casting, can be readily extracted therefrom.
In order to maintain the surrounding sand mass in a sufficiently compacted state, it is customary to place it under a pressure differential by subjecting it to atmospheric or superatmospheric pressure from above, via the open top of the mold, while generating (if necessary) a partial vacuum within the mold to hold the sand particles firmly together.
With certain types of casting, e.g. those of T or H profile in which a broad horizontal web or flange shields lower areas of the pattern from the exerted pressure, this unidirectional compaction of the flowable mass is unsatisfactory and may result in an objectionable deformation of thin-walled parts of the pattern leading to corresponding shape deviations of the finished casting. Conversely, the lack of sufficient density of the particulate mass at shielded locations may allow the molten metal to expand beyond the boundaries of the vaporizing pattern and to penetrate into the mass so as to produce undesirable excrescences on the surface of the casting. If the pattern is originally encased in an antibonding layer or coating designed to prevent adhesion between the metal and the surrounding sand, that layer could be locally ruptured by the penetration of the metal flow into the insufficiently compacted mass, thus further impairing the finished article.